1. Field of the Invention
The present invention relates to a chain tensioner that includes a tensioner body having a cylindrical plunger bore with one open end, a cylindrical plunger slidably inserted in the plunger bore, and biasing means accommodated inside an oil pressure chamber formed between the plunger bore and the plunger such as to freely expand and contract and to urge the plunger in a projecting direction, and that maintains appropriate tension of a chain.
2. Description of the Related Art
It has been common practice to use a chain tensioner for maintaining appropriate tension of a chain. For example, a chain guide mechanism has been known, which slidably guides, by means of a guide shoe, a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room, and which uses a chain tensioner to press a pivoting chain guide having the guide shoe to maintain appropriate tension.
A chain guide mechanism is configured as shown in FIG. 4, for example, wherein a pivoting chain guide G1 and a fixed chain guide G2 guide an endless timing chain CH passing over a drive sprocket S1 of a crankshaft and a pair of driven sprockets S2 and S3 of cam shafts inside an engine room.
The fixed chain guide G2 is fixed in position in the engine room with two mounting shafts B1 and B2, while the pivoting chain guide G1 is attached such as to be pivotable around the mounting shaft B0 in the plane in which the timing chain CH runs in the engine room.
A chain tensioner 500 presses the pivoting chain guide G1 and thereby maintains the tension of the timing chain CH at an appropriate level as well as reduces its vibration.
The known chain tensioner 500 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 5, a tensioner body 510 having a cylindrical plunger bore 511 with one open end, a cylindrical plunger 520 inserted in the plunger bore 511 to freely slide against a cylindrical surface 513 of the plunger bore 511, and means for biasing the plunger 520 in a projecting direction from the plunger bore 511.
The biasing means is formed by a coil spring 540 accommodated inside a cylindrical recess 521 in the cylindrical plunger 520 and compressed between the plunger and a bottom part 512 of the plunger bore 511.
Oil is supplied from an oil supply hole 514 formed in the plunger bore 511, so that an oil pressure chamber 501 formed between the plunger bore 511 and the plunger 520 is filled with the oil, which oil urges the plunger 520 in a projecting direction. A check valve 550 (schematically shown as a check ball) stops the oil from flowing out from the oil supply hole 514.
As the plunger 520 thereby reciprocates, the oil flows through a small gap between the plunger 520 and the plunger bore 511, and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger 520.
In such a chain tensioner, in the event of tension in the timing chain or engine oil pressure building up excessively during use, there were worries that the pressure of the oil pressure chamber could become too high, because of which noise or vibration problem could arise, or the timing chain could be damaged.
To prevent such a situation, chain tensioners provided with a relief valve for releasing the pressure of the oil pressure chamber when it reaches or exceeds a predetermined limit have been known (see Japanese Patent Application Laid-open Nos. 2002-327810, 2011-226534, 2002-130401, 2002-235818, and 2006-125430, etc).